1. Field of the Invention
This invention relates generally to cardiovascular diagnostic and therapeutic catheter systems, and more particularly to such systems that perform three or more functions--including measurement of cardiac output, blood oxygen saturation, and oxygen consumption.
2. Prior Art
One major thrust in the recent prior art of cardiovascular catheter systems is represented by a group of U.S. patents assigned variously by Lieber, Cooper and Estes to American Hospital Supply Corporation--particularly including U.S. Pat. Nos. 4,328,806, 4,329,994, 4,329,993 and 4,407,304.
These patents introduce and explain the great medical importance of multiple-function cardiac catheters (with pacemakers, balloons, etc.) and the extreme desirability of reducing the outside diameter of such a catheter--and, consequently, the number of lumens in such a catheter.
These patents also chronicle the major difficulty which workers in this field encountered before 1980 in trying to effect such reductions: "[I]n order to achieve multiple functions in a cardiac catheter of optimum size, it has generally been considered necessary to compromise the performance capabilities of such a catheter."
The patentees sought to avoid such compromises by carrying electrical conductors or even a gas pathway (for balloon inflation or pressure monitoring) across the boundary septum between two lumens within a catheter. They allocate different functions to different segments of one lumen (and sometimes of two lumens) within the catheter. Thereby, in effect, they make double use of a single lumen.
It is not our intention to criticize unduly the innovations described in these patents, which are considered significant and valuable. In our view, however, the approach adopted by Lieber et al. to minimizing the number of lumens in a cardiovascular catheter is itself characterized by undesirable compromises.
First, that approach involves quite elaborate, expensive techniques for forming an aperture in the septum between lumens, and for installing (or forming in situ) one or more plugs or like structures in the same region. Some of these structures must contain pressure sensors or like devices. The lumens and septa in question are quite tiny, and these mechanical modifications are necessarily fussy, time consuming and, at least as we see it, susceptible to error--with potentially disastrous results.
Second, the Lieber et al. approach inherently leaves a substantial disruption in the catheter structure. Whether the catheter is locally weakened or strengthened is beside our point, which is that the catheter symmetry and the rough isotropy of its materials are substantially disrupted, leading inevitably to problems in operation within the human cardiovascular system.
Such a catheter perhaps may not actually fail in a gross mechanical sense--e.g., tearing away adjacent to the plug. It seems to us very likely, however, that some unacceptable incidence of balloon deflation will occur due to small air leaks around the pressure sensors or other devices that are mounted in the plugs. In short the solution offered by these patents appears to us unacceptable on grounds of questionable inherent reliability as well as high cost.
It is very desirable to achieve high efficiency in the use of a limited number of lumens, so that several diagnostic and therapeutic functions can be performed using a catheter of very small diameter. Yet it is also essential to achieve this goal without introducing additional points of weakness, either mechanical or economic.
Another group of prior patents that may be related to the present invention is granted variously to Shaw, Sperinde, Goldring and Miller, assignors to Oximetrix, Inc. These patents disclose catheter instrumentation and related methods for measuring oxygen saturation in blood.
The instruments monitor optical scattering at two or three different wavelengths, through optic fibers emplaced in the catheters. It is known that such measurements in effect complement the slightly older catheter-effected measurements of cardiac output (by thermodilution) and of pressure at several points in the heart and adjacent blood vessels, as well as balloon functions and pacing.
In particular, the oxygen saturation in the pulmonary artery presents an ideal average value of venous oxygen saturation. This value may be subtracted from an independently measured value of arterial oxygen saturation--which is obtainable straightforwardly by other, conventional techniques--to find the "arterial-venous difference" or change in blood oxygen saturation "across" the body considered as an oxygen-consumption system.
This arterial-venous difference multiplied by the cardiac output (and corrected for hemoglobin concentration and a dimensional constant) then yields the rate of oxygen consumption by the body.
Prior catheter systems for measuring both oxygen saturation and cardiac output (and thus oxygen consumption) have suffered from either the undesirably large diameter of the catheters employed--as explained in the Lieber patents--or from the high costs and unique potential failure modes of the Lieber et al. lumenal-crossover catheters.
Apart from crossover catheters, it has been regarded among persons skilled in the art of cardiovascular catheter technology as essentially impossible to provide a full complement of cardiovascular diagnostic and therapeutic functions in a catheter of 7 or 7.5 French. (The "French" is a customary unit of measure for catheter and needle diameters, one French being equal to a third of a millimeter.) Yet that catheter size is considered the largest permissible for most adult patients.
In view of these limitations as distinct teachings of the prior art, modern proposals to build small but extremely reliable multipurpose commercial catheters have been regarded by persons skilled in the art as essentially unfeasible.
(By "multipurpose" catheters we here mean catheters capable of not only oxygen-consumption measurements but also balloon functions, pacing, and pulmonary-artery pressure monitoring, and in some cases carrying a strength member of Kevlar.RTM. fiber or the like.)
It nevertheless remains extremely desirable, for several reasons, to provide just such catheters. The alternative, very unsatisfactory, is to perform repetitive catheterizations with different catheters, attempting to obtain a full set of data. Cardiovascular diagnostic data, however, should be time-correlated, and they cannot be with serial measurements using different catheters.
Considerable diagnostic difficulty results. This diagnostic difficulty is compounded, to put it mildly, by the inability of many heart patients to stand up to a regimen of testing that is thus protracted.
A final reason, which is really reason enough itself, is in human terms: the extreme aggravation and discomfort undergone by the typically feeble and unwell heart patients being tested and treated. For them this type of testing and therapy is a vital necessity, but at the same time can be a thoroughly miserable experience.